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Pot studies suggest regular use is bad for teen brains

A close look at the under-25 age group shows cognitive decline, poor attention and memory and decreased IQ among those who regularly smoke pot -- defined as at least once a week.

View of a cripin type marijuana leaf inside a greenhouse in La Cominera, department of Cauca, Colombia. A new study suggests regular marijuana use can have significant negative effects on the brains of teenagers and young adults.(Photo: LUIS ROBAYO, AFP/Getty Images)

WASHINGTON — Teenagers and young adults who frequently use marijuana may be hurting their brainpower, according to studies about pot and adolescence presented today at the American Psychological Association's annual convention.

A close look at the under-25 age group shows cognitive decline, poor attention and memory and decreased IQ among those who regularly smoke pot, defined as at least once a week, says Krista Lisdahl, director of the brain-imaging and neuropsychology lab at the University of Wisconsin-Milwaukee.

"It needs to be emphasized that regular cannabis use, defined here as once a week, is not safe and may result in addiction and neurocognitive damage, especially in youth," says a study she co-wrote in the June issue of the journal Current Addiction Reports.

Lisdahl says recent moves toward legalization and decriminalization of marijuana as well as increases in youth use have focused new attention on studies such as hers and others seeking to know more about the impact on youth and their developing brains.

"The adolescent period is a sensitive period of neurodevelopment," she says.

Overall, marijuana use begins in the later teens, around age 16 or 17, peaks in the early 20s and drops off between ages 23 and 25, says Lisdahl.

"Is it a coincidence that use significantly goes down at 25 when the brain is at its full maturation? I don't think so," she says.

Lisdahl says recent studies show increases in marijuana use among high school seniors and young adults. And brain-imaging studies of these regular marijuana users have shown significant changes in brain structure, especially among teens. Brain imaging shows abnormalities in the brain's gray matter — which is associated with intelligence — have been found in 16- to 19-year-olds whose pot smoking increased in the previous year, she says.

A study co-written by Bettina Friese, a research scientist at the Pacific Institute for Research and Evaluation in California, analyzed data from 17,482 teenagers in Montana and found that pot smoking was higher in counties where larger numbers of people voted to legalize medical marijuana in 2004.

"People don't perceive it as a very harmful substance, and these community norms translate to teens," she says. "From the teen study, they do reference legalization: 'If it was that bad a drug, they wouldn't be trying to legalize it.' "

But psychologist Alan Budney, of Dartmouth College, (who works in treatment) says marijuana now is likely a more dangerous product and may mean greater chances for addiction since some legalized forms have higher levels of tetrahydrocannabinol, or THC, the major psychoactive chemical.

"Unfortunately, much of what we know from earlier research is based on smoking marijuana with much lower doses of THC than are commonly used today," he says. "All we know so far is that more people are showing up in the ERs with adverse effects. We've only seen a little bit of it with marijuana, but now we're seeing more of it."

Budney worries that teen pot use is "much, more troublesome" because teens are more vulnerable to the negative consequences of overuse.

"It is just as hard to treat cannabis addiction as it is to treat alcohol addiction," he says.